Atheists are, for the most part, evidentialists. That is, they think that whether or not belief in a divine being is epistemically acceptable will be determined by the evidence. I intend to treat “evidence” in a broad sense including a priori arguments, arguments to the best explanation, inductive and empirical reasons, as well as deductive and conceptual premises. (Also note that one could be an evidentialist theist.) The evidentialist theist and the evidentialist atheist may have a number of general epistemological principles concerning evidence, arguments, implication in common, but then disagree about what the evidence is, how it should be understood, and what it implies. They may disagree, for instance, about whether the values of the physical constants and laws in nature constitute evidence for intentional fine tuning, but agree that whether God exists is a matter that can be explored empirically.
Many believers are non-evidentialists, however. They will deny in one way or another that the acceptability of God belief depends upon evidence, reasons, or arguments. Faith based belief in God, or prudential belief, as examples, will fall into this category. The evidentialist atheist and the non-evidentialist theist, therefore, may have a number of more fundamental disagreements about the acceptability of believing a proposition despite inadequate or contrary evidence (faith), the epistemological status of prudential grounds for believing, or the nature of God belief. Their disagreement may not be so much about the evidence, but about the legitimate roles that evidence, reason, and faith should play in human belief structures.
Justifying atheism, then, will end up being a battle fought on several fronts. There are the evidential disputes over what information we have available to us, interpreting it, and deciding what it implies. But the non-believer will also need to consider the issues and make some larger decisions about epistemic policies generally—the roles of argument, reasoning, belief, and religiousness in human life. The atheist often finds herself not just arguing that the evidence indicates that there is no God, but defending science, the role of reason, and the necessity of basing beliefs on evidence. Theism by contrast is wildly popular, and as a result of people’s general affection for religious belief, such justifications of belief and of one’s epistemic policies are rarely demanded of the believer in a like manner.
For all of these projects, the atheist and the theist need to be clear on the terms that are being used. If one does not believe in God and the reasonableness of belief is in contention, then we must be clear about what sort of God it is that does not exist. A person’s grounds may render atheism about one sort of god reasonable, but not another. Reasons for concluding that the Christian God does not exist may not be sufficient for concluding that no Omni-God exists.
A useful way to divide up the territory here is to think of God (capitol “G”) as an all powerful, all knowing, and all good or morally perfect being. The major monotheistic religious traditions seem to share at least this much in their characterizations. Beyond that definition we can identify the Christian God, or the Muslim God, as an omni-being that also possesses the particular features of that religious tradition. The Christian God, for example, has the three omni-properties (by many accounts), but also sent his only son for the salvation of human kind, rewards belief and piety with eternity in heaven, punishes sinners, etc. An argument or a set of reasons that purport to show that the omni-God does not exist, or that the Christian God does not exist is, therefore, no small matter, particularly since several billion people on the planet currently believe in that sort of being.
Human religious traditions are populated with examples of other beings who are not all-powerful, all-knowing, and morally perfect. See 500 Dead Gods. These beings are most likely not omni-beings, but they have clearly played a comparable role as a divine object of worship in people’s lives. Nevertheless, non-believers have argued that atheism about these lesser beings is reasonable too, albeit on the basis of justifications that are usually different in kind from the justifications of atheism about the omni-God or the Christian God. One’s reasons for rejecting divine beings that are live, relevant, and familiar possibilities are probably different from the reasons that one lacks a belief in some culturally and historically remote being like Zeus, Guangchengzi, Binbeal, or Nin-man. Someone might be an atheist about the Muslim God and about Guangchengzi, but for very different reasons.
If any sort of theism is going to be reasonable, then there is a burden on the believer to account for the general differences, evidential or otherwise, between her belief in a god and her disbelief in these and other gods that she does not accept, whether they are proximate or remote culturally. Often, believers have not met this burden. Consider Zeus believers, their belief, their reasons, and the context of their belief. Given the epistemic, cultural, psychological, and anthropological similarities between their belief in Zeus and many modern believers’ beliefs in their gods, at the very least a significant shadow of doubt is cast on the uniqueness of modern belief. It would seem that we can reasonably conclude that the modern versions of god do not exist for the same reasons that we have reasonably concluded that Zeus or Binbeal do not exist. A useful exercise for the believer and the non-believer is to reflect on the reasons why they do not believe in Zeus, Guangchengzi, Binbeal and try to determine what the general epistemic principles are that are at work in these cases. For most believers, there are far more gods that they do not believe in than gods that they do. A serious challenge is to explain why they are different on non ad hoc, question begging, or fallacious grounds.
There is a frequent complaint against atheism that arises from the wide range of religious ideas, practices, and god conceptions that we find in humans. There are too many god ideas out there and there is too much that we don’t know for anyone to draw any sort of strong conclusion about the non-existence of a divine being. In order to address this criticism, it is useful to consider the various characterizations of God and gods that have been given as target descriptions with specific properties that distinguish them. We can represent these different accounts as occupying different regions of god space—with the omni-conception at the top, or logical limit of this space. The atheist can consider the various god hypotheses in turn, reflect on the evidence that we have that might recommend them, conclude that the arguments are insufficient in favor of that god, or conclude that there is adequate counter evidence to show that no such god exists, and then move on to other conceptions. If atheism about broad categories of god descriptions is reasonable, then broad categories of god space will be empty. As this project proceeds, there will be less god space where a real divine being could exist.
The familiar refrain of “You can’t prove a negative”might lead someone to think that there will always be some description of a god, some region of god-concept space that we have not or cannot consider, so wide-atheism (see Some Varieties of Atheism, Wide Atheism: There Are No Gods Whatsoever) will never be justified. But at some point, these protestations become disingenuous. At some point, the intellectually honest and constructive course of thought should be to see the larger implications of our inquiries and move on. As the probabilities and plausibilities dwindle for Zeus and Binbeal, the project of trying to sustain belief in them amounts to foolish, backward looking foot-dragging. Humanity should be ready to move on. As the gaps for the God of the gaps shrink, we should be prepared to not be enslaved by the concept and abandon it as it becomes more and more difficult to reconcile the idea with everything else we know.
Many believers see responding to the atheist’s criticisms of the tenability of the God idea merely as a matter of developing, adjusting, and exploring the nature of God. And many of these attempts to salvage the idea are efforts to find a conception of God in god space that is not subject to the problems brought up by the atheist. But these efforts at redefining God are constrained because the account that we settle on must be a being that is worthy of worship, deserving of our adulation, awe, respect, and love. Zeus was not. The tantrum throwing, jealous, genocidal God of the Old Testament was not. So the believer is trapped between the broad range of objections presented by the atheist, and the need to give a suitably god-like account of god. And the atheist does not accept that there are any regions of god space where a being like that can still exist given the evidence.